The invention relates to a method of optically writing and subsequent reading and/or erasing information in a recording plane of an optical record carrier having at least two recording planes and a guide plane. A guide beam focused at a spot(guide focus) in the guide plane is used during writing, and at least one write beam focused to a writing spot(write focus) in the recording planes, the guide focus and the write focus being formed by one objective system and the guide focus being held in the guide plane by means of a focus error signal generated by the guide beam.
A method of the type mentioned in the opening paragraph is described in Japanese Patent Application 63-234418. In accordance with this method an objective system converges a guide beam to a guide focus on a guide plane in a record carrier. A focus servosystem controls the objective system in such a way that the guide focus remains in the guide plane in spite of possible excursions of the record carrier. A read or write beam, or generally a scanning beam, is focused by the objective system on a recording plane to be written or read, which plane is parallel to the guide plane. For this purpose the read/write focus of a read/write beam formed by said objective system must be displaceable with respect to the guide focus in the longitudinal direction, i.e. in the direction of the optical axis. Starting from a reference position of the scanning focus, which reference position is equal to the desired position of the guide focus, this is realised by displacing the radiation source supplying the scanning beam along the optical axis over discrete distances which match the distances between the recording planes.
To be able to use the known method with a so-called passive longitudinal adjustment of the scanning focus, the different recording planes of the record carrier must be very accurately parallel to the guide plane within the focus depth of the objective system, because otherwise the scanning focus is not always located in a ,recording plane to be scanned. A multilayer record carrier having such a high degree of parallelism of the layers is difficult to manufacture and is consequently expensive. Moreover, during writing, the scanning focus should accurately follow a given track in a recording plane to be scanned, while during reading the scanning focus should accurately follow the written information tracks. Said Japanese Patent Application 63-234418 does not reveal how this so-called transversal positioning of the scanning focus must be realised.